Polyolefins such as polyethylene and polypropylene may be prepared by particle form polymerization, also referred to as slurry polymerization. With reference to FIG. 1, in this technique, feed materials typically including a monomer and a catalyst are fed to a loop reactor 100, and a product slurry containing solid polyolefin particles in a liquid medium is taken off or withdrawn from the reactor 100.
With reference to FIGS. 2 and 3, in a loop polymerization operation, a fluid slurry is circulated around the loop reactor 100 using one or more pumps 102, typically axial flow pumps having impellers 200 disposed within elbow sections 104 of the reactor 100, the impellors having hubs 208 mounted on impellor shafts 202 that extend through the walls of the elbows 104 and are supported by one or more bearings 300. In some implementations, the bearings 300 are ball bearings that are coated by a lubricant introduced through a lubrication tube 204. In other implementations, the impellor shaft 202 is supported by a sleeve bearing 300 that is coated by a lubricant flowing axially along the impellor shaft 202. Still other implementations use other types of bearing(s) and lubrication. Note that the term “bearings” is used herein to refer generically to all suitable bearing and lubrication implementations, unless the specific context requires otherwise.
The fluid slurry is typically inhibited from reaching the bearings 300 by one or more mechanical seals 302 mounted in a seal housing 312. However, there is a danger that some of the fluid slurry and polymer solids might enter the space 306 between the impellor 200 and the seals 302, or the fluid might even leak past the mechanical seals 300 and react to form polymer solids in the space 304 between the seals 302 and the bearings 300, and so cause damage to the seals 302 and/or to the bearings 300.
Often, the reactor system 100 is configured to introduce the catalyst into the fluid slurry near one or more of the axial flow pumps, so that the action of the axial flow pump mixes the catalyst with the slurry as rapidly as possible. Unfortunately, this configuration can lead to higher than average catalyst concentrations in the immediate vicinity of the pump, and an enhanced likelihood that polymer solids may form in the space between impellor hub 208 and the seals 300, between the seals 300 and the bearings 302, or in a space within the bearing(s).
Since there is substantially no formation of polymer solids in the absence of the catalyst, it is of primary importance to exclude the catalyst from the spaces 304, 306 between the bearings 300 and seals 302 and between the seals 302 and the impellor hub 208. Typically, this is accomplished by providing a pressurized flow of a flushing fluid 308 through a flushing tube 206 into the space 306 between the seals 302 and the impellor 200, thereby forcing the catalyst out and away 310 from the impellor 200. Usually, the flushing fluid comprises one or more of the feed materials, but is devoid of catalyst. In similar implementations, for example when using a sleeve bearing, the flushing fluid typically flows down the shaft tube to the bearing(s).
However, it can happen from time to time that the flow of the pressurized flushing fluid 308 fails, for any of a number of reasons. If this happens, the reactor 100 must be stopped, and frequently it is necessary to service the bearings 300 and seals 302 if damage has occurred. For a typical polymer reactor operating in a commercial environment, such a loss of production while the reactor 100 is stopped can be very costly.
What is needed, therefore, is an apparatus that will at least temporarily exclude catalyst and polymer solids from the bearings and seals of an axial pump in a polymer loop reactor in the event of a failure in the flushing system.